Studies On Preparation And Pervaporation Property Of MFI-type Zeolite Membranes

MFI-type zeolite membranes have attracted researchers' attention because of its well-structured 3D channels and good thermal stability and perfect hydrothermal stability.MFI-type zeolite membranes have extensive applications.They can be used in membrane catalytic reactor and inductor.They can also be used in gas or liquid separation equipment,corrosion-resistant and low dielectric constant materials.However,the separation factor of MFI-type zeolite membranes now is mostly around 50,it should be improved for commercialization.Usually we use secondary growth method to prepare MFI-type zeolite membrane.In order to conform to industrialization requirement,we need to adopt simpler and environment friendly method to prepare membranes.In this dissertation,low-cost macroporous aluminum tubes are used as support.Alkali metal ions are added in the synthesis solution,resulting in improvement of separation performance of membranes.Alkaline induced interface in-situ hydrothermal synthesis method has been adopted to fabricate high quality MFI-type zeolite membranes.(1)Effects of sodium ions on the separation performance of MFI-type zeolite membranesThe improvement of sodium ions added in the synthesis solution is verified for fabrication of high quality MFI-type zeolite membranes.The effects of sodium ions on separation performance,growth and hydrophobicity of MFI-type zeolite membranes are thoroughly characterized by scanning electron microscopy(SEM),X-ray diffraction(XRD),energy dispersive X-ray(EDX)spectroscopy,infrared(IR)spectra,thermogravimetric(TG)analysis,water contact angle test,pervaporation and gas permeation test.It is found that small amounts of sodium ions can result in denser and more hydrophobic membranes with higher pervaporation performance.Large amounts of sodium ions will retard the nucleation and bring about the formation of gel particles,hence resulting in lower pervaporation performance.For separation of 5 wt%ethanol feed at 60 ?,the separation factor of 76 is achieved at the Na+/Si ratio of 0.1,while it is only 52 without sodium ions in the synthesis solution.The role of K+ ions was close to that of Na+ ions.Different alkali metal ions and alkali earth metal ions will cause different effects.Nucleation in the synthesis solution is severely retarded by adding even small amounts of Cs+ ions like large amounts of Na+ ions.Therefore,the role of small amounts of Cs+ ions is close to that of large amounts of Na+ ions.Adding CaCl2 will lead to Ca(OH)2 precipitation in the synthesis solution during aging,thus the separation performance is poor.(2)Alkaline induced in-situ synthesis of MFI-type zeolite membranesIn this dissertation,we proposed a modified method of in-situ hydrothermal synthesis of MFI-type zeolite membranes.Alkaline solution is dipped on the support,rubbed and dried to be distributed uniformly.Neutral synthesis solution(fumed silica,TPABr,H2O and alkali metal salt)is used to in-situ hydrothermally synthesize the membrane on the surface of support.Furthermore,we investigated the effects of concentration of alkaline solution,alkaline coating condition,the synthesis solution composition and synthesis time systematically.We also put forward the mechanism of alkaline induced in-situ crystallization.Alkaline on the surface of the support can offer nucleation site and promote growth of crystals and membranes.With increasing concentration of alkaline,membrane weight and membrane denseness increase,the separation performance increase.When the concentration of alkaline solution is 1.5mol·L-1,the separation factor of MFI-type zeolite membrane is 66 and a flux of 1.03 kg·m-2·h-1 is achieved.Continuously increasing the alkaline concentration leads to stable membrane weight and separation performance.Potassium ions promote the dissolution of high-condensed silica source and growth of crystals and membranes.Other alkali metal ions such as sodium ions have similar effect.However,the effect is worse than potassium ions.During the process of synthesis,neutral solution first contacts with alkaline on the surface of support and support channels near the interface,which can provide nucleation site.Many little crystals form on the support surface and channels nearby.Then,the crystals grow gradually and intergrow to become a dense membrane layer.Afterwards,a c-oriented layer forms on the top of dense layer,the membrane layer becomes denser with increasing synthesis time.Finally the membrane is formed and gain good separation performance.Neutral synthesis solution is used in this method.We can also get nearly neutral solution after synthesis,which is environmental friendly.The in-situ synthesis is simple and easy to operate.It can also be used on the surface of defective supports to get high quality separation membrane.Therfore,the modified in-situ crystallization method has a promising industrial application future.